1. Field of the Invention
The present invention generally relates to a method and apparatus for retaining a workpiece within a semiconductor wafer processing system, and more particularly, to a method and apparatus for providing a chucking voltage to an electrostatic chuck such that the chuck optimally retains a semiconductor wafer.
2. Description of the Background Art
Electrostatic chucks are used for retaining a workpiece, such as a semiconductor wafer, upon a pedestal within a semiconductor wafer processing system such that the wafer can be processed. Although electrostatic chucks vary in design, they are all based upon the principle of applying a fixed voltage to one or more electrodes embedded in the chuck to establish an electric field between the chuck and the wafer. The electric field induces opposite polarity charges to accumulate on the wafer and the electrodes, respectively. The electrostatic attractive force between the oppositely polarized charges pulls the wafer toward the chuck, thereby retaining the wafer upon the chuck. In a Coulombic type chuck, the magnitude of the retention force is directly proportional to the potential difference between the wafer and the chuck electrodes. In a Johnsen-Rahbek type chuck, where the chuck material has a relatively low resistivity and charges migrate from the electrodes to the chuck surface, the magnitude of the retention force is directly proportional to the potential difference between the wafer and the chuck surface.
The chucking voltage that provides the best process results is empirically determined by processing a number of "dummy" wafers at a variety of chucking voltages. The chucking voltage that produces the best results is then repeatedly used to process wafers. During processing, each wafer is retained using the same fixed chucking voltage. Variations in the process parameters that may effect chucking force, chuck leakage current, backside gas cooling efficiency and the like are not taken into account while processing a wafer. As such, one chucking voltage is used to fit all chucking situations. As shall be discussed next, this one size fits all mentality can lead to processing anomalies, intermittent wafer sticking or substantial helium leaking.
In semiconductor wafer processing equipment, an electrostatic chuck forms a portion of a pedestal assembly. The pedestal assembly contains various components for heating or cooling the wafer, providing wafer bias, providing plasma power (cathode electrode) and routing power to the electrostatic chuck. The electrostatic chuck is used for clamping wafers to the pedestal during processing. Since the materials and processes used to process a wafer are extremely temperature sensitive, temperature control is an important aspect of wafer processing. Should the wafer material be exposed to excessive temperature fluctuations resulting from poor heat transfer during processing, performance of the wafer process may be compromised resulting in wafer damage. As such, the pedestal generally forms a heat sink or heat source as used in etching, physical vapor deposition (PVD) or chemical vapor deposition (CVD) applications. To optimally transfer heat between the wafer and pedestal, a very nearly uniform electrostatic force should be used in an attempt to cause the greatest amount of wafer surface to physically contact a support surface of the chuck and contact the surface with a uniform force.
During wafer processing, while the chucking voltage is substantially constant, the chucking force retaining a wafer varies considerably during a given process sequence or recipe. Generally, a nominal, fixed chucking voltage is applied to the chucking electrodes to provide a large nominal retention force that, as the chamber environment varies, the wafer will remain relatively stationary. Although the wafer is stationary, the heat transfer characteristics vary considerably. For example, as a process recipe is performed the chamber temperature and pressure are varied. If the voltage applied to the chucking electrode is fixed, such variations in chamber environment will cause the force retaining the wafer to fluctuate. Variability in the chucking force produces variations in the contact area between the chuck and wafer resulting in variations in the heat transfer from the wafer to the chuck as well as variations in the backside gas (e.g., helium) leak rate from beneath the wafer.
Also, chucking force variations as a wafer is being processed can cause friction between the wafer and the chuck surface that generates particulate contaminants that contaminate the backside of the wafer. Additionally, excessive chucking force can increase the time required to release the wafer after the process is complete (dechucking time). In order to minimize the dechucking time, thereby maximize throughput, it is desirable to apply the minimum chucking force required to achieve good heat transfer. To achieve maximum throughput of wafers that are all uniformly processed, it is also desirable to ensure that the chucking force used from wafer to wafer is uniform as different wafers are processed.
Wafer dechucking is generally accomplished by applying an oppositely polarized voltage compared to the chucking voltage or a similarly polarized voltage at a lesser magnitude as the chucking voltage to remove residual charges from the wafer and chuck. Such dechucking methods are well known in the art and, for example, are described in commonly assigned U.S. Pat. No. 5,459,632 issued to Birang et al. on Oct. 17, 1995. The methods used to dechuck a wafer generally apply a fixed dechucking voltage that is proportional to the magnitude of the chucking voltage. The same chucking and dechucking voltages are used for oxide wafers as well as silicon wafers or any other type of wafer. Such uniform chucking and dechucking voltages can result in wafer "sticking" after a dechucking voltage has been applied. Such sticking is especially problematic in Johnsen-Rahbek type chucks.
As mentioned above, in a Johnsen-Rahbek type chuck, the chuck body is fabricated from a relatively low resistivity material, e.g., aluminum nitride, that enables charges to migrate from the chuck electrodes to the surface of the chuck. Consequently, a small current flows through the wafer at contact points between the chuck surface and the wafer. This current flow, as expected, varies with the resistance of the wafer backside contact points to the chuck surface. As such, an oxide wafer, having a high resistivity surface, conducts very little current. While a silicon wafer, having a low resistivity surface, conducts a substantial current. If a single chucking voltage is used for all forms of wafers, some wafers (e.g., silicon wafers having a higher conductivity) would be chucked with a different force than other wafers (e.g., oxide wafers having a lower conductivity). An increase in chucking force increases the contact area between the wafer and chuck surface and results in increased leakage current flow, and vice versa for decreased chucking force. If an excessive leakage current flows through a wafer, the wafer can experience electron emission which creates a charge imbalance that is not easily removed from the chuck surface. Such an imbalance results in a residual charge remaining on the wafer after a dechucking voltage has been applied. This residual charge is the root cause of wafer "sticking". However, since various wafer types can handle various amounts of leakage current, merely limiting the chucking voltage to a value that ensures a "safe" leakage current in all wafer types results in various types of wafers being chucked with widely varying chucking forces. As such, some wafers may be inadequately chucked and other wafers may be excessively chucked.
Therefore, it is desirable to determine optimal chucking parameters for a wafer and chuck a wafer to achieve the optimal parameters by adaptively controlling the chucking voltage.